Residues or wastes from a petroleum refinery are dispersions of oil and water where variable amounts of these two immiscible liquids are stabilized by finely divided solids such as sand, slime, high carbon content fuels, etc. Such dispersions are not easily susceptible to be broken by the usual techniques. Those and other residues have been the object of several discarding techniques at considerable costs. Incineration is an expensive process requiring highly costly, sophisticated equipment. Landfarming is a technique where the slow bacterial action leads to the final discarding.
U.S. Pat. No. 3,917,564 teaches mixing a petroleum residue containing solids and water (what is known as sludge) and using the so-obtained mixture to quickly quench the coke in the delayed coking process. The technique taught in this U.S. patent is designed to the production of fuel grade coke.
U.S. Pat. No. 5,223,152 teaches that waste streams containing oil, water, high molecular weight fractions, green waxes, coke fines and other solids are produced during the refining of petroleum. These waste streams are generated as a result of the refinery process itself, cleaning processes, maintenance and other such occurrences. It is highly desirable to recover, to the extent possible, the valuable products contained in these waste streams, not only for realizing the value of the products recovered but for minimizing the amount of waste, which must be extensively treated to allow for disposal in an environmentally acceptable manner.
Normally, refinery waste streams are collected and sent to an API separator for initial processing. At the API separator, gravity separates the refinery waste into three layers, a primarily-solids or sludge layer at the bottom, a primarily-water layer in the middle and a primarily-oil layer at the top. The primarily oil layer is skimmed from the top of the API separator and collected in a large surge tank for eventual reprocessing to recover valuable products. The recovered oil or slop oil from this primarily-oil layer is typically composed of 80 to 90 weight percent oil with the remainder being water. Some entrained solids, such as green waxes and coke fines, may be present in the recovered oil but would usually be at very low levels.
Recovered oil normally contains a wide boiling range of hydrocarbon materials. Thus, reprocessing of recovered oil is typically carried out by feeding the recovered oil to a fractionator or distillation column to separate out the various products. For refineries that contain a delayed coker unit it is usually the coker fractionator that is used to fractionate the recovered oil. Conventionally, a recovered oil stream is pumped from tankage through a preheater and then fed to the bottom of the coker fractionator.
However, the re-processing of recovered oil presents operation problems. This is due to the water content of the recovered oil. When water that has been dispersed and stabilized in the oil by the action of the high molecular weight fractions is heated through indirect heat exchange with a warmer product stream or process steam and fed to a fractionator, the water vaporizes. As the water vaporizes it expands with a significant increase in volume. The force associated with this expansion causes pressure surges which can severely damage equipment and severely upset operation of the fractionator.
Such upsets can lead to contamination of the lighter product streams from the fractionator. Heavy boiling range components can be carried up the tower thereby contaminating these product streams. These contaminations then cascade into downstream process units resulting in further contaminations.
U.S. Pat. No. 4,985,131 teaches a process for treating refinery sludge to produce petroleum coke. In the described process, the sludge feed is dried and heated and the light hydrocarbons and coke residue are separated.
U.S. Pat. No. 5,922,189 teaches a process for the refining of petroleum residues and sludge generated by the oil producers, refiners and re-refiners that comprises the steps of heating under vacuum the petroleum residues and sludge by injecting steam or inert gas or both up to a temperature in the range of 360° C. and up to 538° C. and keeping the mixture at that temperature for a short residence time between 1 and up to 6 hours, while vacuum and aspersion are carried out to produce asphalt. Volatile products are condensed to yield fuel; wax oil and can be further processed to yield refined fuel, wax and dewaxed oil. The obtained product is an asphalt, in a reduced time process and without using propane-based distillation processes. However, this U.S. patent does not contemplate the processing of multiphase residues, mainly those residues having high water content, nor the use of a reactor having a gradient of heating, at various temperatures, with collectors and scrubbers for the several kinds of collected gaseous products. This U.S. patent envisages a continuous inert gas feed, while in the present system and process the inert gas is added in the beginning of the operation to create an inert atmosphere, that is later on kept inert by gas generated by the process itself.
On the other hand, U.S. Pat. Nos. 4,618,735, 4,781,796, 5,865,956 and 5,847,248 are directed to they conversion of municipal wastes for producing gaseous and liquid hydrocarbon products in the absence of oxygen, basically using low-temperature conversion processes or LTC. Such processes are based on the processing of a dry feed of municipal wastes or other residues, this rendering not viable the application of such technology to the situation of petroleum residues or from other sources having unlimited contents of water, oil and solids.
Thus, in spite of the approaches envisaged by the technique, there is still the need of a system for treating multiphase residues having an unlimited content of water, oil and solids aiming at obtaining hydrocarbon fractions and other useful products, said system comprising a tubular reactor provided with a fixed pitch screw conveyor where the multiphase residue is heated under reduced pressure and in the presence of an inert gas, the heating being carried out in distinct temperature zones with a first zone of evaporation of free and emulsified water and elution of light hydrocarbons, a second zone of thermal desorption and a third zone of mild pyrolysis, the various hydrocarbons fractions being collected in condensers while the solids are separated for post-treatment and industrial use, such system and associated treatment process being described and claimed in the present application.